One of the most common mistakes made by gemstone buyers is selecting a gem based on carat weight rather than size. Most people are familar with diamonds, and have a good idea of the size of a 1 carat brilliant-cut diamond. But if you buy a 1 carat sapphire, you are likely to be disappointed if you expect it to be the same size as your 1 carat diamond. The sapphire will appear substantially smaller.

Same Size - Different Weight

6 mm Diamond 0.82 ct

6 mm Sapphire 1.48 ct

There are two reasons for this. First, sapphire is a denser material than diamond. Since the different gemstone varieties have different chemical compositions, they vary with respect to their density or specific gravity. Diamond is not a particularly dense material as far as gemstones go, and a number of gemstones have a higher density, including sapphire, ruby, spinel, garnet and zircon. On the other hand, some gems such as tourmaline, emerald and quartz have a substantially lower density than diamond, and will appear larger in the same carat weight.

The second reason why a 1 carat sapphire will appear smaller than a 1 carat diamond has to do with cutting style. Diamonds are cut to maximize brilliance, while colored gems are cut to maximize color. Diamonds have a larger "face-up" size, since they are cut with a large crown and a small pavilion. Colored gems like sapphires are cut with different proportions; smaller crowns and larger pavilions. This approach results in a richer color.

If you are careful to check the size when purchasing a gemstone, you will never go wrong, since a 6 mm round stone is always 6 mm, regardless of its weight. A 6 mm round ideal-cut diamond will weigh about 0.80 carats. But a 6 mm round sapphire may weigh anything from 1.2 to 1.4 carats.

If you are buying a colored gem to fit a jewelry setting, it is especially important to buy by size rather than carat weight. A jeweler can make a custom setting to fit any size of gem, but manufactured jewelry settings come in standard or calibrated sizes. You will find calibrated sizes for many different gemstone shapes, including round, oval, pear, marquise, octagon, square and heart.